The present invention pertains to a high molecular weight polyethylene which is suitable for the manufacture of films, and fibres with good properties. The invention also pertains to these tapes and fibers, to methods for manufacturing the polyethylene, and to objects manufactured from said tapes and fibers.
It has long been known that polyethylene with a high molecular weight is suitable for manufacturing films and fibers with good properties. A problem, however, has always been the processing of these materials.
In the art, this problem has been solved by the provision of processes involving the use of solutions. U.S. Pat. No. 5,503,791 describes a polyethylene film manufactured by extruding a solution of a polyolefin in a first solvent, followed by cooling, the removal of the solvent and stretching of the film. A disadvantage of the thus obtained film is that it always contains a certain amount of residual solvent, which may detrimentally affect the properties of the film. In general the amount of solvent present in solution of gel-cast films is at least 100 ppm. Further, the recovery of solvent is highly uneconomic.
EP 292 074 describes a method for processing polyolefins with a Mw of more than 400.000, in particular between 500.000 and 10.000.000, a low entanglement density, and a Mw/Mn of less than 10, preferably less than 5 at a temperature below the melting point of the polymer using a process auxiliary agent. The polyolefin is first compressed in the presence of a processing aid, and then subjected to solid state processing. The processing aids mentioned in this reference are polymer solvents. In the Example 17 wt. % of decaline is used.
U.S. Pat. No. 5,106,558 describes a method for the continuous preparation of a polyethylene having high strength and high modulus which comprises the steps of mixing 100 parts by weight of an ultra-high molecular weight polyethylene powder having an intrinsic viscosity of 5-50 dl/g in decalin at 135° C. with 2-50 parts of a liquid organic compound having a boiling point higher than the melting point of polyethylene, feeding the resulting mixture between a pair of rollers, and subjecting the mixture to compression moulding and drawing.
WO 2009/007045 and WO 2010/079172 describe UHMWPE films having a tensile strength of at least 2.0 GPa, a tensile energy to break of at least 30 J/g, a Mw of at least 500 000 gram/mole, and a Mw/Mn ratio of at most 6. A new high molecular weight polyethylene has now been identified which can be processed in a solvent-free manner to form films and fibers with good properties. The polyethylene of the present invention is characterised by the following properties: A number average molecular weight Mn of at least 2.0*105 g/mol, a weight average molecular weight of at least 2.0*106 g/mol, a Mw/Mn ratio of above 6, and a strain hardening slope of below 0.10 N/mm at 135° C.